Plasmas are often used to activate gases placing them in an excited state causing enhanced reactivity. The gases may be excited to produce dissociated gases containing ions, free radicals, atoms and molecules. Excited gases are used for numerous industrial and scientific applications, including processing solid materials such as semiconductor wafers and powders, other gases, and liquids. The parameters of the dissociated gas and conditions under which the dissociated gas interacts with a material to be processed by the system vary widely depending on the application. For example, atomic fluorine is used to etch materials such as Si, SiO2, W, and TiN. Atomic oxygen is used to remove photoresist or other hydrocarbon materials. Ionic and atomic hydrogen can be used to remove oxides of copper and silicon. The amount of power required in the plasma for dissociation to occur may vary depending on, for example, gas flow rates and the specific application for the plasma source.
Plasma sources generate plasmas in various ways. For example, plasma sources generate plasma by applying an electric field in a plasma gas (e.g., O2, NF3, Ar, CF4, N2, H2, and He) or a mixture of gases. Plasma sources may generate a plasma using a DC discharge, microwave discharge, or radio frequency (RF) discharge. A DC discharge generates a plasma by applying a potential between two electrodes in a plasma gas. A microwave discharge generates a plasma by directly coupling microwave energy through a microwave-transparent window into a discharge chamber containing a plasma gas. An RF discharge generates a plasma either by electrostatically or inductively coupling energy from a power supply to a plasma. Electrostatically coupled plasmas, usually having higher ion bombardment energies than inductively coupled plasmas, are typically used in applications where higher ion energy is preferred, or where higher ion energies do not cause any deleterious effects. Inductively coupled plasmas are used in applications where surface bombardment by high energy ions is not desired or where a high plasma density is needed. Inductive and electrostatic coupling often simultaneously occur in an inductively-coupled plasma device, as there is usually a high electric voltage applied to the induction coil and the coil is in close proximity to the plasma. Electrostatic shields may be used between an induction coil and a plasma to prevent or reduce electrostatic coupling. However, electrostatic shields often reduce coupling efficiency between an induction coil and plasma, causing RF power to be wasted and impedance matching to become more difficult. Plasmas are often contained in vessels that are composed of metallic materials such as aluminum, dielectric materials such as quartz or sapphire, or a combination of metallic and dielectric materials.
Conventional plasma sources can be expensive and difficult to maintain. Certain vessel shapes are more expensive and difficult to manufacture. Certain vessel materials, such as sapphire or Y2 O3, are available in limited shapes or geometries. Further, plasma vessels are vulnerable to erosion during operation and must be replaced periodically in conventional systems. This is particularly true when highly reactive ions and atoms of hydrogen, nitrogen, oxygen, fluorine and chlorine are involved.
A need therefore exists for effectively exciting a gas with a plasma in a manner that minimizes the erosion or maximizes the life of the plasma vessel, minimizes the cost associated with manufacturing and using the plasma source, and maximizes plasma stability and gas excitation efficiency.